Conventionally, the following exposure unit is known as an exposure unit for forming an electrostatic latent image in an image forming apparatus. The exposure unit includes a laser diode that forms an electrostatic latent image by projecting light onto a photosensitive member, a photodiode that detects the quantity of light emitted from the laser diode, a CPU that generates an output voltage corresponding to the present emitted-laser-light quantity detected by the photodiode, a reference voltage generating portion that generates a reference voltage corresponding to a set print density, and an emitted-light quantity control unit that makes a comparison between the detection value of the emitted-light quantity and the output voltage generated by the CPU and controls the quantity of light emitted from the laser diode in accordance with a quantitative relationship therebetween.
In this image forming apparatus, when the emitted-light quantity of the laser diode is controlled to increase from a state in which the laser diode does not emit light to a state of reaching an emitted-light quantity corresponding to the reference voltage, the CPU increases the output voltage two-step by two-step for a while for which the detection voltage detected by the photodiode becomes equal to a pre-set voltage, and increases the output voltage one-step by one-step for a while for which the detection voltage exceeds the pre-set voltage and reaches a targeted voltage (see JP-A-3-5865, for example).